1. Field of the Invention
The present invention relates to a compound superconducting wire comprising a core including compound superconducting material such as Nb.sub.3 Sn, a diffusion-preventing layer surrounding the core wire, and a stabilizing layer surrounding the diffusion-preventing layer and made of stabilizing material such as Cu, which has a high electrical conductivity, said diffusion-preventing layer preventing the elements of the compound superconducting material from diffusing into the stabilizing material. The invention also relates to a method of manufacturing the compound superconducting wire.
2. Description of the Related Art
Various compound superconducting wires are known, among which is an Nb.sub.3 Sn multi filamentary superconducting wire. This multi filamentary superconducting wire is manufacture in the following way. First, an Sn wire is inserted into a Cu tube. The Sn wire and the Cu tube are inserted into an Nb tube. The Sn wire, the Cu tube, and the Nb tube are inserted into a Cu tube. The Sn wire, the Cu tubes, and the Nb tube are stacked together and subjected to reduction of area by means of a swaging machine, and/or a drawing machine into a rod having a regularly hexagonal cross section. This rod and some other identical rods made by the same method are inserted into a Cu tube used as a stabilizing tube. The rods and the Cu tube are stacked together and subjected to reduction of area by means of a hydrostatic extrusion machine and the drawing machine into a wire. This wire is heat-treated, whereby each Sn wire and the Cu tube surrounding the Sn wire are alloyed together, and the tin (Sn) in this alloy reacts with the Nb tube surrounding the Cu tube, forming a Nb.sub.3 Sn superconducting layer on the Nb tube. Thus, a compound, multi filamentary superconducting wire is manufactured.
FIG. 1 is a sectional view of such a multi filamentary superconducting wire. As this figure shows, this superconducting wire includes six rods embedded in a Cu rod 4 as a stabilizing member. Each of the rods comprises a Cu-Sn core 1, an Nb.sub.3 Sn layer 2 surrounding the core 1, and a diffusion-preventing layer 3 made of Nb and surrounding the Nb.sub.3 Sn layer 2. The stabilizing member allows a fast passage of an electrical current when the Nb.sub.3 Sn 2 undergoes transition, from superconducting state to normally conducting state, thereby protecting the tube 2 against burning damages and maintaining the Nb.sub.3 Sn layer stable.
Copper (Cu) which is used as the material of the stabilizing member 4 has an very high residual resistance ratio (RRR) of 200 to 800; its low resistivity .rho. is about 1.times.10.sup.-8 [.OMEGA..multidot.cm] at 20[K]. To make matters worse, the electrical resistance of Cu decreases when Sn of the Cu-Sn core 1 diffuses into the stabilizing rod 4 during the heat treatment.
The diffusion-preventing tube 3 prevents Sn from diffusing into the rod 4 indeed, but cannot completely prevent Sn from diffusing into the rod 4 during the heat treatment. No measures are taken to prevent niobium (Nb), i.e., the material of the tube 3, from diffusing into the stabilizing rod 4. In the case where Nb.sub.3 Sn, i.e., the material of the tube 2, contains titanium (Ti) or tantalum (Ta), Ti or Ta inevitably diffuses into the rod 4 during the heat treatment. When Nb, Ti, or Sn diffused into the stabilizing rod 4, the RRR of the rod 4 decreases to 1 to 20, and the resistivity .rho. of the rod 4 is about 1.times.10.sup.-7 to about 1.times.10.sup.-6 [.OMEGA..multidot.cm] at 20[K]. That is, the copper (Cu) into which Nb, Ti, Ta or Sn has diffused has electric resistivity about ten to hundred times higher than that of pure copper. As the result of this stability of the compound superconducting wire is reduced.
To suppress the diffusion of impurities into the stabilizing member 4, the unfinished product is heat-treated at low temperatures and for a relatively short time. However, when the temperature and time of the heat treatment are reduced, however, the formation of the Nb.sub.3 Sn layer 2 is impaired, resulting in a superconducting wire whose critical current density is insufficient.
The superconducting wire, whose stabilizing member 4 is made of Cu, is disadvantageous for its low mechanical strength. Generally, the wire is strengthened by a tension member made of stainless steel. It requires a complex process to assemble the wire and the tension member into one composite member. The manufacture cost of this composite member is high, and the space factor of this member is low.